1. Field of the Invention
This disclosure pertains generally to a detection circuit and, in particular, to a method and apparatus for detecting differential signal levels using a circuit having a self-induced DC offset.
2. Description of the Related Art
FIG. 1 is a circuit diagram of a conventional differential signal detection circuit 100 described in U.S. Pat. No. 6,194,965 B1 issued to Kruczkowski et al. on Feb. 27, 2001 (hereafter the ""965 patent) and assigned to Cypress Semiconductor Corp.
The circuit 100 includes a differential amplifier stage 102 and an output circuit 104. The output circuit 104 includes a differential buffer 142. The differential amplifier stage 102 includes a logic circuit 130 composed of a differential OR gate 132.
The differential amplifier stage 102 of circuit 100 uses four matched differential amplifiers. One of the four matched differential amplifiers is composed of resistors R1, R2, and R5; field-effect transistors (FETs) Q1 and Q2, and a current source 12 to bias the transistors Q1 and Q2. The other three matched differential amplifiers in differential amplifier stage 102 have the same structure but are made from other components. The top two of the matched differential amplifiers make up a pair circuit 106 and the other two make up a pair circuit 108.
The circuit 100 requires external references VCM_HI and VCM_LO, to perform the signal detect process. The current source 11 and the resistors R5, R6 create common mode voltage VCM_HI in the pair circuit 106, while the current source 15 and the resistors R11, R12 create another common mode voltage VCM_LO in the pair circuit 108. Four comparisons are made to perform the signal detect process in circuit 100, one for each matched differential amplifier. Signals taken from nodes Z_N, Z_P, Y_N, and Y_P from the pair circuits 106 and 108 form the inputs to the differential OR gate 132.
Embodiments of the invention improve upon the conventional circuit by eliminating the need for the common mode voltages VCM_HI and VCM_LO, which decreases noise susceptibility. Embodiments of the invention also improve upon the conventional circuit by reducing circuit area and power consumption.
Embodiments of the invention provide a differential signal detection device and an indication circuit. A particular embodiment includes at least two differential amplifiers that are purposely mismatched so that one leg of each amplifier is different than the other leg in the same amplifier. This gives the differential amplifiers a non-symmetric characteristic.
Other embodiments of the invention provide a data bus signal level detection system. The system includes first and second input terminals coupled to first and second signal lines of a data bus, and first and second differential amplifiers coupled to both the first and second input terminals. An indication circuit produces signals based upon the output from the first and second differential amplifiers. The differential amplifiers each have a first leg with a first set of components and second leg with a second set of components, but the first and second set of components do not match.
A method for detecting differential signals is also provided. First and second differential input signals are applied to purposely mismatched differential amplifiers. The differential amplifiers are mismatched because the leg of one branch of the amplifier is non-symmetric when compared to the other leg of the amplifier. The output signals from the differential amplifiers are then compared in a logic circuit that produces an output based on that comparison.